Conventionally, as disclosed in Patent Literature 1, there has been considered as a line light irradiation device that includes a long-size shaped LED substrate having a plurality of LEDs mounted thereon, a casing for accommodating this LED substrate, and a round bar shaped rod lens fixed to the casing for condensing light rays from the plurality of LEDs to a line shaped one. Specifically, a structure for fixing the rod lens positioned above the LEDs is configured of projections corresponding to an outer diameter of the rod lens and buffer members provided between the projections and the rod lens to thereby prevent looseness of the rod lens.
However, the line light irradiation device described in this Patent Literature 1 is to be assembled by inserting the rod lens into the casing along a longitudinal direction while the casing is in a state of fixing the buffer members to the projections. If so, there arises a problem that it is hard to insert the rod lens into the casing due to a slip resistance between the buffer members and the rod lens. This problem becomes more remarkable as a longitudinal size of the casing becomes larger or longitudinal size of the rod lens becomes longer.